Olefins industry is an important basis for the development of the chemical industry. The production of low carbon olefins mainly employs the technical process of the high temperature water steam cracking of naphtha, wherein the reaction needs to be conducted under the condition of 800° C. or more, which is one of the processes consuming relatively large energy in the chemical industry. Recently, the price of the international crude oil keeps increasing, the cost of the raw material for olefin greatly increases, and the olefin corporations face more strict status. At the same time, the requirement in the international market for propylene presents the tendency of greatly increasing, and the product distribution of the traditional water steam cracking process which is dominated by ethylene could also not satisfy the increasing requirement of propylene in the market. The above factors promote the development of new olefin technology. The technology for producing ethylene and propylene by catalytic cracking at a relatively low temperature attracts broad attention. Meanwhile, catalytic cracking may results in a higher propylene yield, satisfying the increasing propylene requirement.
Naphtha is a mixed hydrocarbon product of C4-C12, the composition thereof is mainly saturated alkanes, which accounts for 50-95 wt % of the total compositions. These light hydrocarbons have a low carbon number and a high saturation degree. Currently, the commercial technology of producing low carbon olefins through cracking reaction from these light hydrocarbons is only known as the high temperature steam thermal cracking. Large amount of methane and coke are produced in the reaction. In order to solve the defects of high energy consumption and low raw material utilization, a series of catalytic cracking technologies are developed. Currently, the catalytic cracking technologies for saturated hydrocarbons and the naphtha dominated by saturated hydrocarbons are divided into two types of the fixed bed and the fluidized bed technologies.
In the fixed bed reaction process, the former Soviet Union develops a Kalium-Vanadium Vniios process (USSR Pat 1298240.1987). This catalyst uses potassium vanadate as an active component, α-Al2O3 as a carrier, and oxides such as B2O3 and the likes as an aid. The semi-industry and industry experiments of naphtha catalytic cracking have been accomplished at 800° C. in the presence of steam. The yields of ethylene and propylene in this process are 38% and 14.5%, respectively, and the propylene/ethylene ratio is about 0.4. U.S. Pat. No. 3,767,567 uses Al2O3 and an oxide of any one of CaO, SrO and BaO as a catalyst for the catalytic cracking of naphtha. The reaction temperature is relatively high. With the generation of ethylene and propylene, a relatively large amount of dry gases, CO and CO2 are produced. U.S. Pat. No. 4,172,816 uses Ag-MOR/Al2O3 as a catalyst, and conducts the reaction between 600 to 750° C. The yield of ethylene and propylene reaches 42%. U.S. Pat. No. 6,288,298 uses a silicon phosphorus aluminum molecular sieve SAPO-11 as a, catalyst for naphtha cracking, the light naphtha components cracking at 575° C., where the conversion is 39.2%, and the propylene selectivity in converted products reaches 56%. Patent ZL 02152479.3 of Dalian Institute of Chemical Physics, Chinese Academy of Sciences uses a modified molecular sieve as a catalyst, conducting the catalytic cracking of a naphtha raw material containing 60 wt % of a chain alkane and 30 wt % of a cyclic alkane which is reacted between 600-700° C., and the yield of ethylene and propylene reaches 45-50%.
The process for producing olefins by fluidized bed catalytic cracking disclosed in the Patents mainly uses the high-carbon atom number olefins as the cracking raw materials to conduct the production of low carbon olefins, but the patent technologies using the saturated hydrocarbons as the main cracking raw material is very few. WO099/57085 and WO01/64761 start from the raw material rich in olefin (20-70%), employ fluidized bed and a short residence time (1-10 s), and the raw material contacts with the molecular sieve-containing catalyst to produce C2-C4 olefins under the condition of a catalyst to raw material ratio of 2-10. EP 0109059 discloses a process of converting C4-C12 olefins to propylene. The employed catalyst is ZSM-5 or ZSM-11 molecular sieve with a silicon-aluminum ratio lower than 300, and the reaction is carried out at a space velocity higher than 50 h−1, and a reaction temperature of 400-600° C. The total yield of ethylene and propylene is 36-44%, wherein the propylene yield is 30-40%. U.S. Pat. No. 4,830,728 introduces a fluidized bed catalytic cracking device used for maximizing the olefin yield. This device has two risers, wherein the heavy raw diesel oil is converted in one riser, while lighter olefins or naphtha raw material is cracked in another riser, and the adjustment of the condition for raw diesel oil riser may maximize the production of gasoline and olefins.
The above described catalytic cracking has such features that the alkaline catalytic cracking generally needs to be achieved at relatively high temperature. Although comparing with thermal cracking, the reaction temperature thereof is relatively reduced, it does not completely overcome the problem of high energy consumption and high methane production. Using the acidic molecular sieve catalyst may achieve the cracking of the raw material hydrocarbons at a relatively low temperature, but there is still the problem of system heat supplying.
The utilization of the coupling of different reaction processes is an efficient procedure for reducing the reaction thermal effect. Nowak et al. add C4 hydrocarbon during methanol conversion process to conduct the heat coupling (Appl. Catal. A 50(1989)149-155). At a reaction temperature of 600-700° C., when the molecule ratio of methanol to n-butane is 3:1, the reaction process on the HZSM-5 molecular sieve achieves the thermal neutralization. The coupled cracking of methanol and C6 hydrocarbons and naphtha also shows the promotion effect for the low carbon olefins production. The patent ZL 02152480.7 of Dalian Institute of Chemical Physics, Chinese Academy of Sciences suggests a coupled technical routine of producing low carbon olefins utilizing the catalytic cracking of the organic oxygen-containing compounds and the petroleum hydrocarbons. By coupling the reaction process having exothermic effect, the coupling of proper exothermic reaction of the organic oxygen-containing compounds causes that the cracking of petroleum hydrocarbons turns from a strong endothermic reaction process to a relatively strong or relatively weak endothermic reaction process, and may improve the yield of the low carbon olefins such as ethylene, propylene, and so on.
The methanol reaction and hydrocarbon cracking reaction mainly employ different catalyst systems. The present invention applies a modified ZSM-5 catalyst to the coupled reaction of the both, achieving the methanol coupled hydrocarbons cracking. Comparing with the separated naphtha cracking reaction, the modified ZSM-5 catalyzed methanol coupled reaction has a higher low carbon olefins yield and co-producing aromatic hydrocarbons.